Add shell command for turning on/off WebView multiprocess.

Now that we are removing the WebView Multiprocess Developer Setting we
instead add a couple of shell commands for debugging purposes:
adb shell cmd webviewupdate enable-multiprocess
adb shell cmd webviewupdate disable-multiprocess

We also ensure that multiprocess for WebView is enabled after this
change - even if the user disabled it earlier.

Bug: 36513859
Test: Run WebViewUpdateServiceTest tests.
Test: Ensure disabling/enabling multiprocess through the new adb
commands causes WebViewZygote to be used accordingly, and kills apps
using WebView.
Test: Disable multiprocess on a previous OC-build, then flash a build
with this change in it, and ensure multiprocess is turned on
(multiprocess is currently on by default for all devices).
Change-Id: I923198781777e33848b671ec44f50bbc65b43e24